The discussion in this section is only to provide background information of the present embodiment and does not constitute an admission of prior art.
Unlike the past when radio communication provided a service centered on voice traffic, the number of backhaul links for transmitting hundreds of megabytes per second (Mbps) or more are gradually increasing with the rapid development of third generation/fourth generation (3G/4G) mobile communication for providing a multimedia service. Also, with the advent of next-generation mobile communication such as 5G mobile communication, a necessity of wireless transmission of gigabytes per second (Gbps) or more is increasing such that frequencies of a millimeter wave band which facilitate ensuring a bandwidth of hundreds of MHz or higher are attracting attention. Further, 3G partnership project (3GPP) began a discussion about standardization. In this way, considerable discussions about 5G element technology development and 5G standard technology are taking place among companies and organizations. Here, millimeter waves denote electromagnetic waves having a frequency of 30 GHz or higher (30 to 300 GHz). Currently, 28 GHz, 38 GHz, 60 GHz, 70 GHz, and the like are taken into consideration as frequencies to be used in 5G mobile communication networks.
Such a millimeter wave band shows a larger transmission loss and a lower diffractive feature than an existing 4G frequency band, and thus a beamforming technology for concentrating radio waves toward a desired direction using a plurality of antennas is generally used in wireless transmission.
Meanwhile, 5G mobile communication employing such millimeter waves requires a large number of small cells to cover many shadow areas caused by a high transmission rate and a low diffractive feature. Therefore, in consideration of capital expenditure (CAPEX) and operating expenses (OPEX), a necessity of low-priced small cells increases.
However, when a wired transmission network is used to backhaul data from a small cell to a macro cell, a large amount of cost is required to build the wired network separately from the low-priced small cell.
As a solution to this problem, a self-backhauling technology for separating, while using the same frequency/time resources, a wireless backhaul between a macro-cell base station (BS) and a small-cell BS and a wireless link between a BS and a terminal according to a beamforming technique is attracting attention.
In a general wireless backhaul, a backhauling frequency band between BSs and a frequency band used for a terminal are different to prevent interference. In this case, since it is necessary to assign predetermined frequency resources for backhauling, a frequency capacity of a small-cell BS is reduced. On the other hand, the millimeter wave band enables backhauling in which the same frequency/time resources are used due to high directivity based on beamforming.
However, even when a beamforming-based wireless backhaul is implemented between a macro-cell BS and a small-cell BS, there is a problem of interference between each cell and a terminal.
Since a macro-cell BS transmits and receives radio waves in a relatively larger range than a small-cell BS, there is no significant difference in distance and angle between a small-cell BS and terminals connected to the small-cell BS from the viewpoint of the macro-cell BS, and thus interference occurs even when radio waves are transmitted and received based on beamforming. This problem worsens in a downlink in which the macro-cell BS emits a wireless backhauling wave with high transmission power.
In other words, during beamforming-based wireless backhauling, there is an interference factor for a terminal that receives a downlink signal from a small-cell BS.